By definition there is no such thing as space in between the grades, though a stone can be near or at the border.
Nature doesn't place each diamond in the middle of each grade.
Some must be borderline.
When this happens a human or two at the lab must make the call.
That said, sometimes the diamond's owner strongly disagrees and resubmits the stone and the lab may change the grade.
IIRC, the fine print on the back of grading reports say the lab can be wrong by one grade.
It really always is subjective when the stone is borderline. GIA, in general, seems to be stricter on color. I personally have had a stone that was GIA graded H but AGS graded it as G. But different graders within the same lab could certainly grade a stone up or down. There is no exact line of when the color grade changes..as opposed to age (your example)...and that changes on the next birthday!